Sean Penn, that is – ferocious actor, controversial political figure, reformed photographer puncher and director of the new movie “Into the Wild,” out Friday.

Adapted from the 1996 best seller by Jon Krakauer, the film tells the true story of Christopher McCandless, a young man who, upon graduating with honors from Emory in 1992, gave all his money to charity, got rid of his

stuff and walked away from civilized life. For the next two years, he tramped around the country, seeking the kind of raw, real-life experiences he read about in his favorite books by Leo Tolstoy, Jack London and Henry David Thoreau.

His ultimate pilgrimage was deep into Alaska’s Denali National Park – where, 4 1/2 months later, a group of moose hunters discovered a note on the door of an abandoned bus he’d been using for shelter.

“S.O.S. I need your help,” McCandless had written. “I am injured, near death, and too weak to hike out of here. I am all alone, this is no joke. In the name of God, please remain to save me. I am out collecting berries and shall return this evening.”

He wasn’t, though. His decomposing body was found inside, in a sleeping bag his mom had made for him. He’d successfully survived on his own in the wilderness for almost 16 weeks, with minimal supplies, and was only done in by a relatively minor mistake: He confused a poisonous plant with a similar-looking edible one.

It’s not hard to see why this outsider’s life story would appeal to a dedicated contrarian like Penn. His longstanding reputation for acting brilliance is well rivaled by his ability to piss people off. In earlier days, he seemed to be just another Hollywood egoist, best known for lashing out at paparazzi and extras, and for allegedly assaulting his former wife, Madonna.

But in the past decade, Penn has been channeling his anger into social causes – visiting hospitals in Iraq, helping Hurricane Katrina victims, or denouncing the Bush administration in editorials and (rare) interviews.

As a guy with a lot of obvious antipathy for The System, he was a ready-made reader for Krakauer’s book. In fact, when the author describes his subject, he could easily be talking about Penn himself.

“Chris was not an ordinary kid. He was self-absorbed. He was stubborn. He was impetuous …And the extremely wonderful thing about him is that he would not compromise. He had these extremely high ideals, this sense of moral rectitude. He believed that the purpose of life was not to take the easy path.”

After finishing the book, Penn spent the next 10 years trying to persuade McCandless’ family to let him adapt it. When he finally got the green light, he aimed to direct the film in a way that would do justice to his late hero’s spirit. Everything McCandless did, his star would do – minus the dying part – and anything he asked his star to do, he’d do himself.

It all made for a lengthy, grueling, intimidating, exhilarating adventure of a lifetime, says Hirsch, 22, who previously played a skateboard pro in “Lords of Dogtown” and a drug dealer in “Alpha Dog.”

“One of the things Sean made very clear to me before we started was that everything was gonna be authentic,” he says. “And you know, you say, ‘Oh, yeah, I understand,’ but there’s someplace in the back of your mind where you’re like, ‘Well, it’s still just a movie.’

“I quickly realized,” he says, “that was not the case with this movie.”

Tough to say when that realization first hit home, but it may have been when Hirsch had to kayak the Colorado River rapids, solo, with a minimal skill set and a maximum danger of wiping out. Penn did it first. Hirsch had no choice but to follow.

“I made it all three times in one day, which was really a miracle with my level of experience,” Hirsch says. “Everyone was really shocked, including me. The next day, I just ate it immediately, on the first take.”

Reality sank in further the day he had to eat squirrel.

“They fired it up on a skewer, and it looked just horrifying,” the actor recalls with a grin. “Hair was singed onto the flesh. It smelled awful. And Sean, being the kind of leader that he is, took a bite out of it first. And then he hands it to me and goes, ‘OK, your turn. Roll camera!’ ”

It’s hard not to be reminded, at this point, of Werner Herzog, the famously obsessive director who hunkered down with his actors in the Laotian jungle for “Rescue Dawn,” out this past summer. In one of the movie’s most memorable scenes, Christian Bale and Jeremy Davies chow down on a horde of maggots, which Herzog

did, too, in solidarity.

Hirsch, a fan of that movie, proceeds to do a spot-on imitation of Davies in the scene – which is appropriate, as he also rivaled Davies’ shocking weight drop for that role. The normally 150-ish Hirsch dropped down to 115

pounds to play McCandless at the end of his life, when he was starving and unable to hunt.

Jena Malone, who plays McCandless’ sister Carine, developed a sisterly sixthsense concern about her costar

while he was off Outward Bounding with Penn and the crew. “There were a couple of moments when I would wake up and be worried about him,” she says, “and I would text him and be like, ‘Is everyone OK? Have you been eaten by grizzly bears?’

“I know he pushes himself to the extreme, and Sean pushed him even more. Everything he did on film he actually did, so the line became very blurred.”

Eddie Vedder, who wrote the music for the film (as well as for an earlier movie in which Penn starred, “Dead Man Walking”), echoed that sentiment: “Let’s say you’re standing on the edge of a pool, wondering if you should jump in,” he said. “Sean’s the perfect person to push you.”

But unlike some of the crazier stories about total-immersion directors such as Herzog – who had a famously contentious relationship with one of his lead actors, Klaus Kinski – Penn, in spite of his volatile reputation,

seems to inspire only passionate loyalty in his actors.

Veteran actor Hal Holbrook, who plays a sort of father figure who met McCandless shortly before he left for Alaska, raved about the director: “I didn’t know what to expect because, you know, you hear all this stuff about Sean Penn,” he says. “He’s a gentleman. A gentleman! I’ve never been treated more thoughtfully by any director, ever.”

Thoughtful like a fox, perhaps; the 82-year-old Holbrook got surprisingly physical for a scene in which Hirsch dares his character to climb up a steep hill in the desert. “Oh yeah,” Holbrook says, “I went up three times. Sean would say, ‘You know, we have a stand-in …’ And I said, ‘I don’t need a standin! I can climb a freaking mountain.’

“Well,” he says ruefully, “it was a lot harder than I thought it was going to be.”

That scene took it out of Hirsch, as well. “It was a beast of a hill,” says the actor, who knows from hills – at one point, Penn had him slogging up snow-covered ones in Alaska, shouldering a heavy pack.

“Sean told me to just go animal crazy. So I ran as hard as I could, and I could barely feel my legs when I got to the top. They had to help me walk down, and then we got to the bottom and I just puked everywhere.”

Location shooting took the cast and crew all over the country – South Dakota, Nevada, Arizona, California and Alaska, as well as Mexico. In many of those places, Penn found nonactors who worked a lot better than professionals for resembling – well, real people.

Brian Dierker, a whitewaterrafting instructor from the Grand Canyon, ended up in the movie alongside Catherine Keener as a hippie couple that travels the country in their RV. Artist Leonard Knight and his outsider-art creation at Salvation Mountain, Calif., make an appearance. And when Hirsch wanders the streets

of Skid Row in L.A., Penn shot the people who were actually there instead of clearing them out for a film set.

“On the side of the road were just rows and rows of homeless people, sleeping in their beds,” says Hirsch. “Most people don’t want to see that. But Sean would just find people, like their energy, and put them in the movie.”

As for Hirsch, Dierker has invited him to ride the rapids again, and he’s going. Without a camera crew and Penn, though, it may be anticlimactic.

“Yeah, we were having the time of our lives,” says Hirsch. “We were all on this adventure together. I called Sean a couple months after we were back, and I was like, ‘I miss making this movie.’ He said, ‘Me too.’ “